1. Field of the Invention
The present invention relates to a hollow type planar illuminating device configured to emit illuminating light of uniform luminance distribution from a light emitting surface thereof such as a backlight unit for a liquid crystal display apparatus, for example.
2. Description of the Related Art
In recent years, a light source of a backlight unit for a liquid crystal display apparatus has accelerated a move toward replacement of a cold cathode discharge lamp with an LED. This is because the LED is suitable as an environment-conscious light source without use of mercury that is harmful material or because power consumption of the LED can be significantly small due to recent drastic enhancement of LED light emitting efficiency. A backlight unit having an LED as a light source has been used mostly for a small-sized apparatus such as a cellular phone or mobile terminal until now. However, the backlight unit having the LED as a light source has been increasingly used for a large-sized liquid crystal display apparatus such as a liquid crystal monitor or a liquid crystal TV set of 20-inch or larger.
A backlight unit used in a large-sized liquid crystal display apparatus is required to have higher brightness. Accordingly, a direct type backlight unit having an LED light source beneath a surface emitting portion is generally widely used as a backlight unit for a large-sized liquid crystal display apparatus, as disclosed in Japanese Patent Application Laid-Open No. 2005-316337. However, in the direct type LED backlight unit, LEDs that are point light sources having directivity are used. Therefore, a short distance between a plurality of arranged LEDs and a surface emitting portion causes uneven brightness or color. If such uneven brightness or color is generated in the LED backlight unit, the display quality of liquid crystal illuminated by the backlight unit is degraded. This phenomenon further remarkably appears when high-power LEDs of 1 W class or higher per unit are used as a light source to attain high brightness. On the other hand, when a distance between the LEDs and the surface emitting portion (diffusion plate) is lengthened to reduce uneven brightness or color, a thickness of the whole device increases. For example, in order to achieve a light source having NTSC contrast ratio of 100% or more by using single-color light-emitting LEDs for each of blue, red and green for enhancement of color reproducibility on a liquid crystal panel, it is necessary to ensure color mixing properties; therefore a further increase in the thickness of the apparatus is required. However, such an increased thickness is not preferable, which is against a recent tendency of slimness of a liquid crystal display apparatus.
In addition, a direct type backlight unit using white light-emitting LEDs as a light source has been developed for commercial availability. However, each of the white light emitting LEDs uses blue light emitted by LEDs and yellow light emitted by a phosphor mounted in the vicinity of the chip. Accordingly, when such white light emitting LEDs are used as a light source, the light emitted from the backlight is perceived as white; however, there is posed a problem of generation of an unmatched color, for example, when the light passes through a liquid crystal display panel. Accordingly, there are many problems in application of such a direct type backlight unit using white light emitting LEDs as a light source to a liquid crystal monitor for medical purpose or computer graphics, which require higher color reproducibility.
As a solution for such problems in the direct type backlight unit, there is also known a side light type backlight unit which guides emitted light from an LED light source arranged at an end portion within a device through an end surface of a light guide plate, reflects and diffuses the light on a reflecting surface thereof and emits the light from a light emitting surface on the opposite side. However, in the case of the side light type backlight unit using the light guide plate, when the light emitting surface is enlarged to meet a recent bigger a screen for a liquid crystal display apparatus, it is necessary to significantly increase the area of the light guide plate in the same way as a light emitting surface. Use of such an enlarged light guide plate poses a problem of increasing the overall weight of the apparatus.
Further, in recent years, there is also known a hollow type backlight unit which reflects the light incident from the outside in a hollow light guide space to emit the light evenly from a surface emitting portion and irradiate a liquid crystal panel in the front thereof with the light, as disclosed in Japanese Patent Application Laid-Open No. 2006-106212. The conventional hollow type backlight unit has an advantage that the weight of a device is not remarkably increased even if the area of backlight unit is enlarged to meet bigger screen enlargement of a liquid crystal display apparatus because a member remarkably increasing the weight such as a light guide plate is not employed.
However, in the conventional hollow type backlight unit, when a light collecting lens is used on the light emitting surface side of an LED light source unit to reduce a light emitting angle of the light from the LED and a white-based reflecting sheet excellent in light reflection properties is used as a reflecting surface member, in particular, the light is collected into a flat beam through the light collecting lens, so that the light concentrates in a fixed region of the hollow light guide reflecting sheet. Accordingly, the light spectrally-reflected in the region is applied to a fixed region of the light emitting surface member and thus there is posed a problem of generation of a brighter line in a corresponding region of the light emitting surface.